1. Field of the Invention
The present invention generally relates to a power distribution system and, more particularly, to a method of reducing switching noise in a power distribution system by external coupled resistive terminators.
2. Description of the Related Art
Precisely controlling timing skew is one of major challenges in high-speed digital signaling. Among various reasons that cause the timing skew, power integrity is recently considered to be a principal concern, especially under a requirement of high throughput and low voltage swing. Switching noise is a dominant noise source in a power distribution system. Minimizing this noise will be beneficial to overall power integrity.
When an I/O buffer is switched, it will not only draw energy from the power distribution system in a very short period of time, but also induce a broadband noise onto the power distribution system. The noise level is exacerbated even further when multiple signals make transitions at the same time, which is referred to as simultaneously switching noises. These simultaneously switching noises are basically in phase or nearly in phase and therefore their noise amplitudes can be accumulated, instead of being cancelled. If designed improperly, power/ground planes of the power distribution system form a resonator such that the noise with a frequency closer to a certain resonant frequency can be stored up, causing more severe problems to the power integrity.
One of the most common and greatly accepted solutions to suppressing the noise is placing decoupling capacitors on to the power distribution system; however, equivalent series inductances of wirings used to connect between the decoupling capacitors and the power distribution system limits a possible application to a high frequency regime.
Alternatively, lowing the resonant effect can effectively alleviate the noise accumulation since the noise is more violent upon resonance. The resonance effect on the noise can be avoided by detuning the resonant frequency from an operating frequency or adding more loss to reduce a quality factor of the power distribution system. For example, noise absorption materials are provided at edges of a circuit board to effectively minimize reflection and radiation due to edge discontinuity, especially at a high frequency regime. This method is referred to as “edge termination.” Two types of noise absorption materials have been proposed, i.e. electric and magnetic lossy materials. However, in the case of the electric lossy materials, an undesired leakage current is induced and, in the case of the magnetic lossy materials, it is impossible to achieve a broadband absorption due to a lack of appropriate magnetic lossy materials.